Large scale sand extraction on sand ridges offshore of the Netherlands

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Large-scale sand extraction

on sand ridges offshore of

the Netherlands

Inventory of instruments to predict physical effects of sand extraction on the Zeeland ridges

S. Hommes

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Ministerie van Verkeer en Waterstaat

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Rijksinstituut voor Kust en Zee/RIKZ

Author: S. Hommes

Supervisors: Ir M. Boers (RIKZ)

Prof. dr S.J.M.H. Hulscher (UT) Drs. A. Stolk (RWS, DNZ) Ir H.H. van der Veen (UT) Workdocument: RIKZ/OS/2004.105W

Project: K2005*ZEEBODEM

Programme: KUST2005

Inventory of instruments to predict physical effects of sand extraction on the Zeeland ridges

3 February 2004 Definitive version

Large-scale sand extraction

on sand ridges offshore of

the Netherlands

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Preface

This report is the result of my Master thesis of Civil Engineering at the University of Twente. I did my research at the Rijksinstituut voor Kust en Zee (RIKZ) in The Hague. I would like to take this opportunity to thank some people, who helped me to bring this research to a good ending. First, I want to thank the committee. Marien Boers, my supervisor at RIKZ, for his critical comments and help on the research strategy and the rest of the report and Ad Stolk, my supervisor of Rijkswaterstaat, Directie Noordzee (RWS, DNZ), for his useful contribution from a decision-makers perspective. Furthermore, Henriët van der Veen of the University of Twente for helping me get started with the research and checking the report. Finally, I want to thank Suzanne Hulscher for her difficult, but useful questions and critical comments.

During two weeks of my research, I did some model runs at the University of Twente. I want to thank Pieter Roos and Michiel Knaapen for letting me work with their models and for accompanying me. A highlight of my thesis was the workshop ‘Modelling sand mining on the Zeeland ridges’, which I organized on November 18th_{2003 at RIKZ. I would like to thank all the}

participants of the workshop: Marien Boers, Job Dronkers, Dries van den Eynde, Suzanne Hulscher, Déborah Idier, Michiel Knaapen, Sonja Ouwerkerk, Leo van Rijn, John de Ronde, Pieter Roos, Teun Terpstra, Pieter Koen Tonnon, Henriët van der Veen and Dirk-Jan Walstra, for their presentations, co-operation and enthusiastic contribution to this day. I also want to thank Jan Mulder for leading the day as chairman.

Finally, I want to thank my colleagues at RIKZ, for their sociability and interest in my research during my stay at RIKZ. And last, but certainly not least, I am very grateful for my family and friends, who helped me through rough times of this research period and the rest of my study.

Have fun reading!

The Hague, 3 February 2004 Saskia Hommes

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List of figures

FIGURE 1.1: WATER DEPTH (M BELOW LLWL) ONNCS (TNO, 2003)... 13

FIGURE 1.2: INFORMATION CYCLE (AFTER: KINNEGING, 2001) ... 15

FIGURE 2.1: RUGBY BALL FOR THIS RESEARCH (AFTER: RIJKSWATERSTAAT, 1998) ... 17

FIGURE 2.2: PROCESS INFORMATION CHAIN... 19

FIGURE 3.1: SAND RIDGE PATCHES IN THE NORTHSEA (VAN DE MEENE, 1994) ... 20

FIGURE 3.2: ORIENTATION TIDAL SAND RIDGES (GRAY; ANTI-CLOCKWISE)AND SHOREFACE-CONNECTED RIDGES (WHITE; CLOCKWISE)WITH RESPECT TO TIDAL CURRENT... 21

FIGURE 3.3: BATHYMETRY ZEELAND RIDGES (TNO, 2003)... 21

FIGURE 3.4: BATHYMETRY SAND EXTRACTION AREA ON THE MIDDELBANK,DEPTH IN DM – NAP (MAP FROM RWS, DNZ, 2003) ... 22

FIGURE 3.5:DISTRIBUTION OF HOLOCENE DEPOSITS IN THE YEARS 5500, 3500 AND 1700BEFORE CHR. (ICID, 2003) 23 FIGURE 3.6: MAXIMUM TIDAL CURRENTS AT THE WATER SURFACE (M/S)IN THE AREA OF THE ZEELAND RIDGES;SPRING TIDE (A); NEAP TIDE (B) (FROM KUSTZUID MODEL, 2003) ... 24

FIGURE 3.7: TOP VIEW SKETCH OF VORTICITY PRODUCTION (ANTICYCLONIC = -; CYCLONIC = +) RESULTING FROM A TIDAL CURRENT FLOWING OVER A RIDGE. THE VORTICITY DURING FLOOD (LEFT SIDE OF RIDGE) AND EBB (RIGHT SIDE OF RIDGE), AS WELL AS THE RESULTING (AVERAGED OVER TIDAL PERIOD) CURRENT DIRECTION (DASHED ARROWS)ARE INDICATED. (WALGREEN, 2003) ... 25

FIGURE 3.8: SCHEMATIC VIEW OF THE GROWTH OR DECAY OF BOTTOM PERTURBATIONS THROUGH TRANSPORT OF SEDIMENT (WALGREEN, 2003) ... 26

FIGURE 3.9: SAND RIDGE SYSTEMS ON THE BCSAND NCS (WEBSITEMUMM, 2003) ... 28

FIGURE 4.1: SANDPIT CYCLE (LEFT)AND SCALE GRAPH (RIGHT) (HOMMES, 2004) ... 32

FIGURE 4.2: ENVIRONMENTAL INFORMATION MODEL... 33

FIGURE 4.3: COASTLINE MAP OF SCHOUWEN (RIKZ, 2003)... 36

FIGURE 4.4: EXCAVATION DEPTH PIPELINES DIAMETER > 0.4 M... 37

FIGURE 5.1: DEFINITION SKETCH OF THE MODEL GEOMETRY (ROOS& HULSHER, 2003) ... 40

FIGURE 5.2: DEFINITION AMPLITUDE... 45

FIGURE 5.3: SHELF GEOMETRY (DESWART & CALVETE, 2003) ... 46

FIGURE 6.1: FINITE-ELEMENT METHOD... 54

FIGURE 7.1: KWINTEBANK ON THE BCS; TWO DEPRESSED (RED) AREAS (LEFT); BATHYMETRY (RIGHT) (MAPS FROM THE MINISTRY OF ECONOMIC AFFAIRS, BELGIUM, 2003) ... 59

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List of tables

TABLE 3.1: CHARACTERISTICS ZEELAND RIDGES, HINDER BANKS AND FLEMISH BANKS... 29

TABLE 3.2: CHARACTERISTICS ZEELAND RIDGES AND ENVIRONMENT... 30

TABLE 4.1: COASTAL SAFETY AND MAINTENANCE PARAMETERS... 36

TABLE 4.2: OFFSHORE INFRASTRUCTURE PARAMETERS... 37

TABLE 4.3: NAVIGATION PARAMETERS ... 38

TABLE 5.1: SCENARIOS TWENTE MODEL ... 42

TABLE 5.2: RESULTSAMPLITUDE-EVOLUTION MODEL... 45

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List of abbreviations

2D: Two Dimensional

2DH: Two Dimensional Horizontal 2DV: Two Dimensional Vertical

3D: Three Dimensional

ADCP: Acoustic Doppler Current Profiler BCS: Belgian Continental Shelf

BUDGET: Beneficial usage of data and geo-environmental techniques CSI: Coastal State Indicator

CZM: Coastal Zone Management

DNZ: Directie Noordzee; Directorate North Sea EIA : Environmental Impact Assessment HISWA: Hindcast Shallow WAter Waves LLWL: Low Low Water Level

MCL: Momentaneous Coast Line

MUMM: Management Unit of the North Sea Mathematical Models and the Scheldt estuary

NAP: Normaal Amsterdams Peil NCS: Netherlands Continental Shelf

RBINS: Royal Belgian Institute of Natural Sciences

RIKZ: Rijksinstituut voor Kust en Zee; National Institute of Coastal and Marine Science

RON/MER: Regionaal Ontgrondingenplan Noordzee RON2: Regionaal Ontgrondingenplan Noordzee 2

RWS: Rijkswaterstaat; Directorate-General of Public Works and Water Management

SDN: Stichting De Noordzee

SUTRENCH: SUspended sediment transport in TRENCHes SWAN: Simulating WAves Nearshore

TAC: Total Allowable Catch

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Summary

This Master thesis is accomplished at the Rijksinstituut voor Kust en Zee (RIKZ) as part of KUST2005, which is a morphological research programme of Rijkswaterstaat (RWS). The management question of RWS (Directie Noordzee, DNZ) concerns the effects of large-scale (> 10x106_m3_{) sand}

extraction on the Zeeland ridges. DNZ wants to gain knowledge on the physical effects of sand extraction on the Zeeland ridges. Therefore, the objective of this study is to make an inventory of instruments that can be used to predict the long-term physical effects of large-scale sand extraction on the Zeeland ridges, in order to support the Dutch decision-making process.

The Zeeland ridges is a group of ridges located in front of the coast of the Dutch provinces Zeeland and Zuid-Holland, seaward from the continuous -20 m NAP depth contour line. The characteristics of the Zeeland ridges are: 5-15 m height, 9-39 km length, transverse spacing 3-7 km, oriented 0-20 degrees clockwise with respect to the tide, (partly) covered with sand waves of 2-8 m height. The origin of the ridges is not clear. First, they were considered as drowned dunes, but others concluded that they were partly formed by erosion of older forms and partly by sand accumulation. Furthermore, there are no data that point to migration of the Zeeland ridges. At first sight, the Zeeland ridges seem to be tidal sand ridges. However, the ridges are oriented like shoreface-connected ridges, clockwise with respect to the tide. Except for the fact that the ridges are not connected to the shore. Therefore, no clear conclusion can be drawn on the classification of the Zeeland ridges.

To achieve the objective as formulated, we determine the information need of decision-makers and other stakeholders, using the ‘Rugby ball method’ from ‘Meetstrategie 2000+’ of RWS. An important aid in this process is the definition of a set of Coastal State Indicators (CSI’s); a quantitative concept of the actual state of the system as a basis for objective and transparent decision-making. Each indicator is related to a specific coastal user function. In this research, we restrict ourselves to the user functions: coastal safety and maintenance, offshore infrastructure and navigation. For each of the selected coastal user function, research questions, CSI’s and if possible assessment criteria are formulated. The result of the Rugby ball is the inventory of instruments (objective Master thesis). We selected state-of-the-art models: three analytical (non)-linear stability-type models (Twente model, Utrecht model, Amplitude-evolution model) and four numerical models (Delft3D, SUTRENCH, Telemac, mu-SEDIM) to include in this inventory. The current situation of these instruments on the modelling of sand extraction is investigated and the future possibilities of each model are estimated. The information on the models is gathered through interviews with model developers, literature study and organizing a workshop for model developers. Furthermore, with the Twente model and the Amplitude-evolution model some calculations are done.

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In the inventory of instruments it is indicated for which CSI’s the instruments are applicable and with what reliability. From this inventory it was clear that the a couple of instruments are capable to directly support the decision-making process on a few CSI’s (but only on short-term effects) and on the remaining part of the CSI’s qualitative and quantitative insight in a schematised situation is possible at this point. We conclude that the most important information missing are measurements on sand transport and knowledge on the long-term (morphological) evolution of the sea bottom. The Kwintebank on the Belgian Continental Shelf can form a good test case. From 1979 till 2003, sand was extracted on this ridge. During this period the Kwintebank was also monitored intensively. Since February 2003, there is a temporary extraction closure of three years, because two depressed areas on the ridge exceeded the permitted extraction depth. During the closure period intensive monitoring will take place, to evaluate the natural potential of restoration of the ridge. It would be worthwhile to propose an idealised configuration of the Kwintebank and its depressed areas. Then, we could run models on this idealised case and compare the results in between and the magnitude orders with the field observations.

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Samenvatting

Dit afstudeeronderzoek is gedaan bij het Rijksinstituut voor Kust en Zee (RIKZ) binnen het project KUST2005, een morfologisch onderzoeksprogramma van Rijkswaterstaat (RWS). De beheersvraag van RWS (Directie Noordzee, DNZ) concentreert zich op het effect van grootschalige (> 10x106_m3_{) zandwinning op de Zeeland banken. DNZ wil}

kennis vergaren over de fysische effecten van deze zandwinning. Het doel van dit onderzoek is het maken van een inventarisatie van instrumenten, die gebruikt kunnen worden om de fysische lange termijn effecten van grootschalige zandwinning op de Zeeland banken te voorspellen, om het Nederlandse besluitvormingsproces te ondersteunen.

De Zeeland banken is een groep banken voor de kust van de Nederlandse provincies Zeeland en Zuid-Holland, zeewaarts van de doorgaande –20 m NAP diepte contour lijn. De karakteristieken van de Zeeland banken zijn: 5-15 m hoogte, 9-39 km lengte, afstand tussen banken 3-7 km, oriëntatie 0-20 graden met de klok mee ten opzichte van het getij, (gedeeltelijk) bedekt met zandgolven van 2-8 m hoogte. De oorsprong van de banken is onduidelijk. Eerst werden ze beschouwd als ondergelopen duinen, maar anderen concludeerden dat ze gedeeltelijk gevormd zijn door erosie van oude vormen en gedeeltelijk door aanzanding. Verder zijn er geen data die duiden op migratie van de Zeeland banken. In eerste instantie lijken de Zeeland banken getijdenbanken te zijn. Ze zijn echter georiënteerd zoals kustaangehechte banken, met de klok mee ten opzichte van het getij. Alleen zijn ze niet verbonden met de kust. Kortom, er kan geen duidelijke conclusie getrokken worden ten aanzien van de classificatie van de Zeeland banken.

Om het geformuleerde doel te bereiken, bepalen we de informatiebehoefte van beleidsmakers en andere belanghebbende. Hierbij wordt gebruik gemaakt van de ‘Rugbybal methode’ uit ‘Meetstrategie 2000+’ van RWS. Een belangrijk hulpmiddel hierbij is het definiëren van een set van Coastal State Indicators (CSI’s); een kwantitatief concept van de werkelijke toestand van een systeem, dat de basis vormt voor objectieve en transparante besluitvorming. Elke indicator is gerelateerd aan een specifieke gebruiksfunctie van de kust. In dit onderzoek beperken we ons tot de gebruiksfuncties: kustveiligheid en –onderhoud, offshore infrastructuur en scheepvaart. Voor elk van de geselecteerde gebruiksfunctie worden onderzoeksvragen, CSI’s en wanneer mogelijk beoordelingscriteria geformuleerd. Het resultaat van de Rugbybal is de inventarisatie van instrumenten (doel afstudeeronderzoek). We hebben een aantal state-of-the-art modellen geselecteerd voor deze inventarisatie: drie analytische (niet)-lineaire stabiliteitstype modellen (Twente model, Utrecht model, Amplitude-evolutie model) en vier numerieke modellen (Delft3D, SUTRENCH, Telemac, mu-SEDIM). De huidige situatie van de instrumenten ten aanzien van de modellering van zandwinning is onderzocht en de toekomstmogelijkheden van elk model worden bepaald. De informatie over de modellen is verzameld door middel van interviews met model ontwikkelaars, literatuurstudie en de organisatie van een workshop voor model ontwikkelaars. Verder zijn er met het Twente model en het Amplitude-evolutie model een aantal berekeningen gedaan.

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In de inventarisatie van instrumenten wordt aangegeven voor welke CSI’s de instrumenten toepasbaar zijn en met welke betrouwbaarheid. Uit de inventarisatie blijkt dat een aantal instrumenten in staat zijn om het besluitvormingsproces op een paar CSI’s direct te ondersteunen (alleen op korte termijn effecten) en voor de overige CSI’s is het mogelijk om kwalitatief en kwantitatief inzicht te verschaffen in een geschematiseerde situatie. We kunnen concluderen dat de belangrijkste informatie die mist zandtransport metingen zijn en kennis op het gebied van (morfologische) lange termijn ontwikkeling van de zeebodem. De Kwintebank op het Belgisch Continentaal Plat zou een goede test case zijn. Van 1979 tot 2003 is er op deze bank zand gewonnen. Gedurende deze periode is er ook intensief gemeten. Sinds februari 2003 is de Kwintebank tijdelijk gesloten voor zandwinning, omdat er zich twee depressie gebieden op de bank bevonden die de toegestane winningdiepte overschreden. Gedurende de sluiting wordt er intensief gemeten, om het natuurlijke herstel van de bank te evalueren. Het zou de moeite waard zijn om een geïdealiseerde configuratie van de Kwintebank en haar depressie gebieden te formuleren. Vervolgens kunnen we met de modellen rekenen aan deze geïdealiseerde situatie en de uitkomsten vergelijken met elkaar en de orde grootte met de veldmetingen.

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1 Introduction, Research objective and strategy

1.1 Sand extraction offshore of the Netherlands

The North Sea may be a small, shallow pool compared with oceans, but nevertheless it is teeming with life. Water and sediment are home to a wide variety of plants and animals. The North Sea is also a sensitive ecosystem that is under a great deal of pressure from intense human activities such as fishing, sand and gravel extraction, shipping, oil and gas extraction, tourism and industry. On the Netherlands Continental Shelf (NCS) an average of 30 million m3_{of sand per year is extracted and used for filling sand,}

nourishments and large infrastructure projects. Nowadays only small-scale extractions take place. However, land reclamation projects (like Main Port Rotterdam; Maasvlakte 2) and sand extraction activities for the construction industry may lead to larger and deeper sand pits in the NCS and sand extraction at ridges, both seawards from the continuous –20 m NAP depth contour line. Furthermore, the use of sand resources on land provokes more and more resistance, by citizens and (local) politics. The authorities need information on the morphological effects of large-scale (> 10x106_m3_{) sand pits to make decisions on granting licenses. A judgement}

framework for granting licenses on large-scale sand extraction is however still in development (Rijkswaterstaat, 2003b).

1.2 Framework of the research

Policy on offshore sand extraction

In 1993, the regulations on extraction activities in the Dutch North Sea were formulated in the ‘Regionaal Ontgrondingenplan Noordzee’ (RON/MER). The most important statements in RON/MER were the following: sand extraction is only allowed seaward from the -20 m NAP depth contour line, except for harbour entrances and shipping channels; only deepening up to 2 m is allowed; an area that has been mined once is not allowed to be mined again. In 2004, the ‘Tweede Regionaal Ontgrondingenplan Noordzee’ (RON2) is released. RON2 differs from RON/MER on the following points: it distinguishes between small-scale extraction (< 10 million m3_{) and large-scale extraction (> 10 million m}3_{); for}

large-scale extraction or extraction of an area greater than 500 hectares an Environmental Impact Assessment (EIA) is required; the –20 m NAP depth contour line as landward boundary for extraction is replaced by the continuous –20 m NAP depth contour line; for large-scale extraction it is possible to mine deeper than 2 m, if an EIA shows that this is acceptable (Rijkswaterstaat, 2004; Stolk, 2003). For further details on the policy of offshore sand extraction, we refer to Terpstra (2004).

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KUST2005

KUST2005 (COAST2005) is a morphological research programme of Rijkswaterstaat (RWS), which started in 2000 and lasts for five years. Based on an inventory of the major morphological coastal management problems on different timescales, COAST2005 aims: to contribute to the solution of specific coastal management problems; and to guarantee an improvement in quality of coastal engineering advice and models for the longer term. The specific problems concern four themes. This Master thesis is accomplished at the Rijksinstituut voor Kust en Zee (RIKZ) as part of the fourth theme, which focuses on the effects of (large-scale) sand extraction in the North Sea (Rijkswaterstaat, 2003a). In this research, we focus on the physical effects of large-scale sand extraction on the Zeeland ridges. The Zeeland ridges is a group of ridges located in front of the coast of the Dutch provinces Zeeland and Zuid-Holland, seaward from the continuous -20 m NAP depth contour line (marked by the circle in Figure 1.1).

1.3 Previous research on offshore sand

extraction

Several studies related to the behaviour of navigation channels and sand extraction pits in the North Sea have been done recently and are still ongoing. Van Rijn and Walstra (2002) evaluated and summarised most of these studies, which generally focus on the flow and morphology of extraction pits in the North Sea using 2DV, 2DH and 3D hydrodynamic and morphodynamic models, as used by Delft Hydraulics, Svasek and University of Delft. The University of Twente uses bed instability models. These models have not yet been verified extensively due to the lack of field data. Field data sets at deeper water are almost completely missing (Van Rijn & Walstra, 2002). Hoogewoning and Boers (2001) give an overview of the most important physical effects of offshore sand extraction, more specific: the influence on the flow, sediment transport and morphology. Furthermore, they relate these physical effects to involved user functions. Hoogewoning and Boers (2001) focus on three types of extractions: temporary sand extraction in the nearshore zone, large-scale sand extraction by widening of shipping channels and large-scale sand extraction in specific extraction areas (Hoogewoning & Boers, 2001).

Boers and Jacobse (2000) studied the influences of sand extraction on the Zeeland ridges on the wave conditions along the coast of Zeeland. This study was carried out within the framework of KUST2005. The research contributed to the insight in the possibilities of sand extraction on the Zeeland ridges. The conclusions of this research were that sand extraction

Figure 1.1: Water depth (m below LLWL) on NCS (TNO, 2003)

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1.4 Research objective

The research on sand extraction at the Zeeland ridges started with a management question (‘beheersvraag’) of RWS, Directie Noordzee (DNZ). This management question concerns the effects of sand extraction on the Zeeland ridges. On one hand, we want to determine the user functions in the area and the value(s) of these ridges. This is the topic of the Master thesis of Terpstra (2004). On the other hand, we want to gain knowledge on the physical effects of sand extraction on the Zeeland ridges, the topic of this research. The objective of this Master thesis is:

The main research question is:

Underlying questions are:

Make an inventory of instruments that could be used to predict the long-term physical effects of large-scale sand extraction on the Zeeland ridges, in order to support the Dutch decision-making process.

In what way can instruments support the Dutch decision-making process on large-scale sand extraction in the area of the Zeeland ridges?

Ø What instruments can be used to predict certain physical effects

caused by large-scale sand extraction on the Zeeland ridges?

Ø Do the output parameters of these instruments support the

decision-makers in their decision? If not:

- What information is missing?

- What knowledge is missing?

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1.5 Research strategy

To achieve the objective and to answer the main research question and underlying questions as stated in the previous section, we have to determine what information decision-makers and other stakeholders require. We do this using a strategy, which is developed by RWS and is called ‘Meetstrategie 2000+’. The strategy is developed for users and suppliers of measurements in the water sector, to determine which field measurements are needed for certain projects or goals (Kinneging, 2001). In this Master thesis, we use the strategy to determine which instruments could be used to predict certain physical effects of large-scale sand extraction on the Zeeland ridges. The strategy consists of an information cycle, which is shown in Figure 1.2 (Kinneging, 2001).

The information cycle starts with a management question. To answer this question, we must first determine the information need of the stakeholders. Next an information strategy is designed, with which the information can be collected. The information that is gained has to be processed in the suitable format and delivered to the managers, who give feedback on the obtained information. Finally, the information need can be adjusted when needed and the cycle starts all over again. Note that the cycle can be run several times and on different levels of detail before the final goal is achieved.

Management and -policy

The information cycle starts with a management question, in this thesis the question of RWS (DNZ) on the physical effects of large-scale sand extraction on the Zeeland ridges.

Information need: Rugby ball method

The second step in the information cycle is the information need; this is the topic of this research. We determine this need using the so-called ‘Rugby ball method’. This method is explained in Section 2.1 and 2.2.

Information strategy

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Technical innovation

Depending on the mismatch between the information need and the possible techniques to collect the information, it can be necessary to search for new, innovative techniques (e.g. models).

Collect and process information

In these steps the designed information strategy is used to collect the information and the information is processed into the right format. This can consist of just presenting the results, but also combining, integrating and aggregating the information.

Feedback

In the last step of the information cycle the manager, gives feedback on the gathered information.

1.6 Reading guide

In the next chapter, the Rugby ball method is explained and the first two phases of this method are described. In Chapter 3, we investigate the research area, the area of the Zeeland ridges. In Chapter 4, we investigate the information need regarding the problem of sand extraction on the Zeeland ridges. Next in Chapter 5, we describe the current possibilities and limitations and the future possibilities of three analytical models, regarding the modelling of sand extraction on the Zeeland ridges. This is done for four numerical models in Chapter 6. In Chapter 7, we describe the results of the RIKZ workshop and the inventory of instruments. In Chapter 8, the results of this research are discussed. Finally, we draw conclusions and give recommendations for future research in Chapter 9.

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2 Rugby ball method

2.1 Rugby ball method in general

The Rugby ball method is part of the strategy called ‘Meetstrategie 2000+’, which is developed by RWS. The Rugby ball method is used in this thesis to determine the information need regarding a large-scale sand extraction on the Zeeland ridges. In this chapter, we describe the Rugby ball adjusted for this research.

2.2 Rugby ball for this research

In Figure 2.1, the Rugby ball for this research is shown.

Phase 1: Exploration

In the first phase of the Rugby ball, the exploration phase, the basic assumptions of the research are determined, a list of information users is formulated and a process information chain is constructed.

Phase 2: Adjustment

The second phase of the Rugby ball is the adjustment phase. We adjusted the basic information need (Phase 1) during committee meetings with the supervisors of this research.

Phase 3: Results

Current situation Zeeland ridges

It is important to know the current situation before we can predict the influence of interventions, in this research sand extraction, on this situation. Therefore, we investigate the geomorphology, origin, sediments, physical processes and classification of the Zeeland ridges. For the current sand extraction market and policy in the Netherlands, the European legislation

Figure 2.1: Rugby ball for this research (after: Rijkswaterstaat, 1998)

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Specify information need

The information need of RWS (DNZ) and RIKZ is analysed by interviews with Ad Stolk (DNZ) and Marien Boers (RIKZ). Moreover, we use information from previous research, workshops and literature to determine the information need of other stakeholders. Finally, we make use of an indicator-concept to express the information need in quantifiable parameters, so-called Coastal State Indicators (CSI’s).

Concept inventory of instruments & Explore instrument(s)

After an inventory of existing mathematical models, we selected three analytical models and four numerical models, to include in the inventory of instruments of this research. The current situation of these instruments is investigated. In order to explore the current possibilities and limitations of the different instruments, it would be ideal to do model runs with all models. However, it would take far too much time in the scope of this thesis to run all seven selected models. Therefore, we only did some exploratory model runs with two of the analytical models. We chose these models, because of practical reasons. The models were available for use at the University of Twente, where the model developers themselves supplied accompaniment.

Phase 4: Assessment

This phase is accomplished with the organisation of a workshop for model developers at RIKZ. At this workshop the participants gave a presentation on their model. After the presentations, a discussion was held, regarding modelling of sand extraction on one of the Zeeland ridges.

Phase 5: Completion

The result of the Rugby ball is an inventory of instruments, which can be used to predict the physical effects of large-scale sand extraction on the Zeeland ridges. In this inventory it is indicated for which CSI’s the different instruments are applicable and with what reliability.

2.3 Exploration phase

2.3.1 Basic assumptions RWS (DNZ)

The basic assumptions are as follows: the final goal of RWS (DNZ) is to predict the long-term effects of large-scale sand extraction on the Zeeland ridges. This information can be used to decide whether to grant large-scale sand extraction on (these) ridges and under what conditions. Furthermore, this research focuses on the physical effects of sand extraction. The timescale on which we want to predict these physical effects is up to 100 years. However, in this research we do not include short-term effects, like silt plumes caused by dredging.

2.3.2 Information users sand extraction

The information users involved with the topic of offshore sand extraction are decision-makers, like DNZ who give permits on sand extraction, and water managers, like RIKZ who give advice on the effects of sand extraction. Further information users are: model developers, who want to

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2.3.3 Process information chain

A process information chain is an aid to determine who undertakes what activities to achieve certain management goals and which information is needed to realise these goals. In Figure 2.2, the process information chain for this research is given. The process information chain starts with a “management question” of DNZ, who want to know the effects of sand extraction on the Zeeland ridges. With this question they come to RIKZ, where a graduating student (Saskia Hommes) is hired to investigate part of this problem. First, the “management question” is translated into “measurement questions” (research questions). RIKZ formulates parameters (CSI’s; Section 4.2), which they want to determine using different instruments. With these “measurement questions” RIKZ turns to instrument experts, who can advice them on the possibilities and limitations of different instruments. Finally, RIKZ translates these “measurement answers” into “management answers”, which can be used by DNZ in the decision-making process.

2.4 Adjustment phase

To adjust the basic information need (from the exploration phase) committee meetings were held with the supervisors of this thesis: Suzanne Hulscher (UT), Henriët van der Veen (UT), Marien Boers (RIKZ) and Ad Stolk (DNZ). In this phase, we discussed and adjusted the assumptions made in the exploration phase. The results of the committee meetings are processed in this report.

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3 Research area: Zeeland ridges

In this chapter, we describe the area of the Zeeland ridges, which is the research area of this Master thesis. First, the general classification of tidal sand ridges and shoreface-connected ridges is described. After this, we investigate the geomorphology, origin, sediments, physical processes and user functions on the Zeeland ridges. In the last section, we aim to classify the Zeeland ridges.

3.1 Classification ridges

Sand ridge patches are significant features on many continental shelves and coastal regions. Their existence depends on the presence of tidal or other currents capable of moving the sand, and the availability of sand. In this section, we describe the general characteristics of tidal sand ridges and shoreface-connected ridges. In Figure 3.1, the location of several sand ridge patches in the North Sea is shown.

3.1.1 Tidal sand ridges

Nearly all shallow tidal seas, where currents exceed 0.5 m/s and where sand is present, have tidal sand ridges. The sand ridges occur in trains, have an elongation of 8-65 km, and a regular transverse spacing of 1.6-10 km. In general the amplitude of sand ridges is between 2 and 20 m. The height-to-width ratio ranges from 1:200 to 1:300, but observations do not indicate a unique relationship between height and width. There are no indications that tidal sand ridges move. Furthermore, their crests are oriented slightly oblique (angles between 5 and 30 degrees) with respect to the principal tidal flow direction, nearly always in an anti-clockwise sense in the northern hemisphere (Figure 3.2). This suggests that the Coriolis force may be a significant factor in the dynamics of ridges (Dyer & Huntley, 1999). It is discussed that tidal sand ridges are relicts, or even moribund features. A feature is called relict if it is not formed by (present or former) flow-topography interaction. The definition of a relict feature does not exclude that its shape can significantly change due to flows and waves. If a feature is not influenced by the present hydrodynamical regime, it is called moribund (Hulscher et al., 2001).

Figure 3.1: Sand ridge patches in the North Sea (Van de Meene, 1994)

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3.1.2 Shoreface-connected ridges

Shoreface-connected ridges are regular bars, which are connected to the lower shoreface with an oblique angle. Generally, patches of 4-8 ridges are observed in water depths of 5-20 m, located several kilometres offshore. The crests of shoreface-connected ridges are rotated clockwise with respect to the dominant axis of tidal motion, which is mirrored to the sense of direction of most tidal sand ridges (Figure 3.2). Furthermore, overall characteristics of these bed forms are: a height of 1-6 m, a width of 2-3 km, and a spacing between successive crests of 2-6 km, while crest lines can extend for several tens of kilometres. Side slopes of the ridge flanks are normally less than 1o

and the ridges have an asymmetrical profile, i.e. they have a steeper slope on the seaward flank. The shoreface-connected ridges are associated with a temporal scale of decades to centuries and are present on shelves where storms contribute significantly to the mean longshore current, whereas the strength of the tidal current strongly varies over the regions of occurrence. Finally, the ridges migrate several metres (0.5-10 m) per year in the direction of storm-driven currents (De Swart & Calvete, 2003; Hulscher et al., 2001; Van de Meene, 1994; Walgreen, 2003).

3.2 Geomorphology of the Zeeland ridges

In this project, we focus on a group of ridges located in front of the coast of Dutch provinces Zeeland and Zuid-Holland, seaward from the continuous -20 m NAP depth contour line: the Zeeland ridges. The Zeeland ridges (Figure 3.3) consist of: Bollen van Goeree (1), Steenbanken (2), Middelbank (3), Schouwenbank (4), Buitenbanken (5), Schaar (6), Rabsbank (7) and Thorntonbank (8). In Appendix A, a geomorphological map of the Zeeland ridges is given.

Figure 3.2: Orientation tidal sand ridges (gray; anti-clockwise) and shoreface-connected ridges (white; clockwise) with respect to tidal current

Figure 3.3: Bathymetry Zeeland ridges (TNO, 2003)

1 2 1 6 5 5 4 3 8 7

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Most northwards, west of the islands Voorne-Putten and Goeree-Overflakkee, two large ridges are located: the Bollen van Goeree. These ridges are orientated in the WSW-ENE-direction, the distance between them is several kilometres, and the crest-trough height is 5 to 10 m and their length 29 and 39 km. The width of these ridges, which we define as half of the distance from the middle of the through on one side of the ridge to the middle of the other trough, is equal to 2-3 km. The ridges are located just seawards from the continuous -20 m NAP depth contour line. The Steenbanken, Middelbank and Schouwenbank are located west of Schouwen-Duiveland and Walcheren. The orientation of these ridges is in SW-NE-direction, the distance between them is around 3 km, and the crest-trough height is more than 10 m and their length 22-30 km. The width of the ridges is rather uniform (2-3 km). The ridges are located just seawards from the continuous -20 m NAP depth contour line. In Figure 3.4, the bathymetry of a sand extraction area on the Middelbank is shown. More offshore of the Schouwenbank, at a depth of -30 m NAP two large sand ridges are located: the Buitenbanken. These ridges are SW-NE oriented, the distance between them is around 3 km, and the crest-trough height is around 10 m and their length around 38 km. The width of the ridges is also rather uniform (2-3 km). Directly south of the Buitenbanken, three smaller ridges (in length) are located, the Schaar, Rabsbank and Thorntonbank. The Thorntonbank and Schaar are SW-NE oriented, the Rabsbank is SSW-NNE oriented. Their crest-trough height varies from 5 to 10 m and their length is 9-16 km.

The whole area, except for a nearshore zone of 5-14 km, is covered with sand waves. The height of these sand waves varies from 2 to 8 m (Figure 3.4). In general, the spatial scale of sand waves is smaller than that of sand ridges; their typical wavelength is between 100-800 m. The general amplitude of the sand wave patterns is of the order of 5 m. In most observed cases, the sand wave crests are almost perpendicular with respect to the major spring tidal current. Sand waves often have asymmetric shapes. Tidal sand ridges and sand wave fields sometimes partly overlap, sand waves fields occur on active ridges. Several authors have proposed estimates for the migration of sand waves. These vary between several metres per year up to 100 m per year. These results should be interpreted with care, due to the uncertainties in spatial location (Hulscher et al., 2001). The location of the sand wave fields in the area of the Zeeland ridges can be found in Appendix A.

Figure 3.4: Bathymetry sand extraction area on the Middelbank, depth in dm – NAP (map from RWS, DNZ, 2003)

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3.3 Origin of the Zeeland ridges

The origin of the ridges in the Southern Bight of the North Sea, the Zeeland ridges, is not clear. Baak (1936; in Houbolt, 1968) considered the Zeeland ridges as drowned extensions of the barrier chain (dunes). These Old Dunes are shown in Figure 3.5. Houbolt (1968) concluded that the Zeeland ridges were in part erosional forms and were not formed by sand accumulation only. Furthermore, Houbolt draws the conclusion that the Zeeland ridges are nearly stationary. There are indeed no data on record that point to a significant displacement of the Zeeland ridges (Van Veen 1936 in: Houbolt, 1968). Laban and Schüttenhelm (1981) concluded that several of the Zeeland ridges are built on top of or are leaning against a core consisting of older deposits at least in part of (early) Atlantic age. These older deposits, provisionally named ‘initial ridges’, appear to be elongated in shape. Finally, we are not aware of any research on the Zeeland ridges after the research of Laban and Schüttenhelm (1981). In Appendix B, more details on the researches on the origin of the Zeeland ridges are given.

3.4 Sediments on the Zeeland ridges

The sea bottom of the North Sea in the area of the Zeeland ridges mostly consists of sand. The surface sediment of the Zeeland ridges consists of moderate coarse sand (diameter = 210-300 mm) and very coarse sand (diameter = 300-420 mm). In general, sand ridges consist of sand, with coarser sand on the top and finer sand on the bottom of the ridges. In the troughs, gravel (diameter = 2-63 mm) and silt (diameter < 63 mm) can be found. In Appendix C, the grain size of the sediment in the first metre is shown for the area of the Zeeland ridges.

Figure 3.5:Distribution of Holocene deposits in the years 5500, 3500 and 1700 before Chr. (ICID, 2003)

blue = sea

orange = barrier chain with Old Dunes pale green = river deposits

grass green= marine sand deposits dark green = marine clay deposits

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A

B

3.5 Physical processes on the Zeeland ridges

3.5.1 Tides

The North Sea is a tidally dominated shelf sea. The tide in the Southern Bight of the North Sea is semidiurnal with a period of around 12 hours and 25 minutes. Three tidal waves enter the North Sea: one through the English Channel, one along the Scottish coast and the Shetland Islands and a third along the Norwegian coast (Appendix D, Figure D.1A). The combined constraint of ocean basin geometry and the influence of the Coriolis force result in the development of amphidromic systems, in each of which the crest of the tidal wave at high water circulates around an amphidromic point once during each tidal period. The tidal range is zero at each amphidromic point, and increases outwards away from it. Interference between the three tidal waves in the North Sea results in two amphidromic points (Appendix D, Figure D.1A). Tidal waves in amphidromic systems are a type of Kelvin wave, in which the amplitude is greatest near the coasts. Kelvin waves occur where the deflection caused by the Coriolis force is either constrained (as at the coast) or is zero (as at the Equator). The typical tidal range in the North Sea varies from 0 to 6.5 m. In the area of the Zeeland ridges the typical tidal range varies, from north to south, from 2.5 to 3.5 m (Appendix D, Figure D.1C). At spring tide the tidal range at Vlissingen is around 4.5 metres and at neap tide it is around 2.5 metres (Appendix D, Figure D.2). We assume that the tidal range in the area of the Zeeland ridges is more or less the same as the tidal range at Vlissingen (Houbolt, 1968; Open University, 1999).

3.5.2 Tidal currents

The strength of the tidal currents in the North Sea is generally moderate (Appendix D, Figure D.1B), in the area of the Zeeland ridges it is around 0.75 m/s (1.5 knots). During spring tide maximum tidal currents of around 1.0 m/s can occur and during neap tide the maximum lies around 0.6 m/s (Figure 3.6). However, during storms much greater velocities can occur. The residual currents resulting from the tidal movements are shown in Appendix D, Figure D.1D. The influence of meteorological conditions on the tidal currents is difficult to determine, the more so as also weather conditions at a greater distance (e.g. off the Norwegian coast) may affect the horizontal water movement near the Dutch coast. The principal meteorological conditions which give rise to changes of the tidal currents are wind fields and to a lesser extent air pressure differences above the North Sea

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3.5.3 Tidal currents over ridges

If a tidal current moves over a ridge, in this case the Zeeland ridges, continuity effects cause an acceleration of the flow over the shallower area. Furthermore, the Coriolis force experienced by the water parcel on the crest is higher than in the deeper water and produces a torque. Conservation of potential vorticity results in the production of anticyclonic (clockwise on the Northern Hemisphere) vorticity over the ridge crest, during both ebb and flood. Vorticity is a measure for rotation and the refraction degree of the flow. A net anticyclonic circulation around the ridge, following the contours, and irrespective of the orientation of the ridge is the result (Figure 3.7, left). In addition, the bottom friction causes a strong deceleration of the flow on shallower areas, which produces torques over the flanks of the ridge (Figure 3.7, middle). This vorticity is advected by the tide. For a ridge oriented cyclonically (anti-clockwise on the Northern Hemisphere) with respect to the tidal flow, anticyclonic residual vorticity is generated over the crest during the tidal period (Figure 3.7, right). In that case both torques enhance each other and the strongest residual circulation is found. Whereas for a ridge of which the crest is rotated anticyclonically (like the Zeeland ridges, see Section 3.7), the Coriolis and frictional torques have opposite directions above the crest and a weak residual circulation is induced.

3.5.4 Waves

The wave climate is rather uniform along the Dutch coast: the dominant wave direction is southwest. Some values of the probability of occurrence (duration in % of time) for waves in deep water are:

· South-west (180o_-270o_{): 15% waves of 1-2 m, 4-5% between 2-3 m,}

1-2% between 3-5m;

· North-west (270o_-360o_{): 10% between 1-2 m, 4-5% between 2-3 m,}

1-2% between 3-5m.

The wave heights mentioned above are significant wave heights (Van Rijn & Walstra, 2002).

3.5.5 Wind

In general, wind can increases or decreases the rates of tidal currents depending on the directions of the wind and currents. To determine the effect of wind in open sea the following rule applies: a current of 2% of the wind velocity should be applied to the tidal currents (thus a wind of 20 m/s generates a current of 0.4 m/s). The wind-generated current is about ten degrees veered with respect to the direction in which the wind blows (a NW-wind generally generates a SE-S-current) (Koninklijke Marine, 2001).

Figure 3.7: Top view sketch of vorticity production

(anticyclonic = -; cyclonic = +) resulting from a tidal current flowing over a ridge. The vorticity during flood (left side of ridge) and ebb (right side of ridge), as well as the resulting (averaged over tidal period) current direction (dashed arrows) are indicated. (Walgreen, 2003)

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3.5.6 Sediment transport

An essential aspect of a coastal system, like the area of the Zeeland ridges is that currents and waves are capable of transporting water and sediment. In the case of sediment, the currents must exceed a certain threshold value before the sediment is transported. The interaction between the water motion and bottom topography through the transport of sediment can result, in some cases, in a positive feedback due to which small perturbations in the bottom start to grow (Figure 3.8). A morphological change in the seabed, e.g. the development of a sand ridge, is the result. A bottom topography that does not change in time under a certain forcing is in equilibrium with this forcing (Walgreen, 2003). For the research area of the Zeeland ridges, no measurements on sediment transport are available. Therefore, we investigate the sediment transport on the Belgian and Netherlands Continental Shelf from literature. We focus on sand transport (and exclude silt transport), because this determines the long-term evolution of sand ridges after mining, which is the topic of this research.

Belgian Continental Shelf

On the Belgian Continental Shelf (BCS), a variety of sediment dynamical studies have been performed. In the course of the BUDGET project, an overview has been produced of all these studies. Most of the data has been re-evaluated and the results were compiled in a synthesis map to characterise the natural sand transport on the BCS. However, using the existing data it is difficult to set up a quantitative sediment balance of the BCS (Lanckneus et al., 2001). Van Lancker et al. (2000) and Van Lancker and Jacobs (2000) (in: Kleinhans, 2002) studied sediment transport on the Flemish banks (Figure 3.9, location C) at water depths of 0-15 m. They found out that the spring-tidal flood current alone can transport sediment of 0.42 mm at least, but sediment of larger grain sizes when the sediment is stirred by waves as well. The coarsest sediments (up to 0.5 mm) with the best sorting are found on the tops of the banks. Vincent et al. (1998) studied the suspended sediment transport under waves and currents in more detail. They did measurements on the Middelkerke Bank (one of the Flemish Banks) during two winters. The wave heights were 1-4.3 m, and were observed to increase the suspension but have no effect on the transport direction. The following sand transport rates were obtained: 0.9 tonnes/m/day (» 200 m3_{/m/year) up to 0.3 m above the bed, along the}

northern steep slope of the Middelkerke Bank. The southern side of the bank appeared to be more wave-sheltered, which explains the lower suspended transport rates of about 0.05 tonnes/m/day (» 10 m3_/m/year).

The transport on the steep slope was mainly in the size range of 100-140 mm (fine sand), which is not significantly present in the bed material, indicating that it was advected by upstream wave- and current action. Excluding the finer component, the transport rates of coarser sand (> 200 mm) at the two sites were similar over the measuring period. Finally, the transport rates are consistent with a timescale of 100-1000 years for the formation of the bank (Kleinhans, 2002; Lanckneus et al., 2001).

Figure 3.8: Schematic view of the growth or decay of bottom perturbations through transport of sediment (Walgreen, 2003)

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Netherlands Continental Shelf

Van de Meene (1994), Van de Meene et al. (1996) (in: Kleinhans, 2002) and Van de Meene and Van Rijn (2000) (in: Kleinhans, 2002) studied the sediment dynamics of the shoreface-connected ridges off the Holland coast at a water depth of 10-20 m. In fair weather, the current-driven bed load transport is dominant and low, whereas in storm, the waves stir up the sediment and the sediment transport is dominantly in the suspended mode, driven by the mean currents. From measurements it could be concluded that both currents and waves rework the seabed to a depth of 0.1-0.2m in the bed. The sediment on top of the sand ridges is coarser and better sorted due to wave action (winnowing of fines) (Kleinhans, 2002). Van Rijn (1995 and 1997 in: Van Rijn & Walstra, 2002) presented estimates and variation ranges of the net annual longshore transport rates at the –20 m depth contour at several stations along the Holland coast, based on state of the art mathematical computations with the TC model. UNIBEST-TC is designed to compute sediment transports and the resulting profile changes along any coastal profile of arbitrary shape under the combined action of waves, longshore tidal currents and wind. The longshore sediment transport (total load) at Noordwijk (about 45 km north of Hoek van Holland) is 35±10 m3_{/m/year, the longshore transport at Scheveningen}

(about 20 km north of Hoek van Holland) is equal to 25±10 m3_/m/year

both in northeast direction. The computed transport rates are compared with transport rates derived from available data of the middle and lower shoreface. Net annual longshore transport rates derived from sand dump sites near Hoek van Holland and about 70 km northwards, near Wijk aan Zee are in the range of 30 to 100 m3_{/m/year for depths between 10 and}

20 m. So, the results obtained from the model computations are in the same order of magnitude (Van Rijn & Walstra, 2002; Website WL Delft Hydraulics, 2003).

Conclusions

From the studies on the BCS and NCS, it follows that the average longshore total transport rate is in the order of 10-100 m3_{/m/year at a depth of}

10-20 m. We assume that the transport in the area of the Zeeland ridges is in the same order of magnitude. However, the transport on the top of a ridge (e.g. Middelkerke bank) can be much higher, in the order of 200 m3_{/m/year. This is probably also the case on the top of the Zeeland ridges.}

These transport rates will change after a large-scale extraction on a ridge has taken place. It is difficult to determine the exact transport rates after a large-scale extraction on a ridge, because there is very little experience on such interventions. However, an extraction pit on a ridge will decrease the flow velocity over the ridge and thereby the sediment transport. The recovery period of a dredged ridge could be of the same order of magnitude as the formation of ridges (decades to centuries), depending on the volume and geometry of the pit,.

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3.6 User functions in the area of the Zeeland ridges

In the area of the Zeeland ridges different user functions are observed (Appendix E, Figure E.2). A pipeline crosses the southern part of the area. Furthermore, there are a couple of cables (in use and not in use) running through the middle of the area. There are also some parts of the ridges on which extractions have taken place. Finally, shipping routes and an anchor area lie around and on the Zeeland ridges. In Appendix E, two maps with the user functions on the NCS and the in the area of the Zeeland ridges are given. For a detailed description of the user functions and ‘values’ in the area of the Zeeland ridges, we refer to Terpstra (2004).

3.7 Classification of the Zeeland ridges

In this section, we aim to classify the Zeeland ridges. First, we compare the Zeeland ridges with the ridges on the BCS. Next we give an overview of the statistics (height, length, orientation, etc.) on the Zeeland ridges and the environment. Finally, we draw conclusions on the classification of the Zeeland ridges.

3.7.1 Comparison with Belgian ridges

The BCS is characterised by a couple of sand ridge systems. In this section, we compare the Zeeland ridges (A) on the NCS to the Hinder Banks (B) and Flemish Banks (C) (Figure 3.9).

Figure 3.9: Sand ridge systems on the BCS and NCS (Website MUMM, 2003) A = Zeeland ridges; B = Hinder Banks; C = Flemish Banks

B

C

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Hinder Banks

About 50 km west of Walcheren a group of sand ridges occurs; the Hinder Banks. These ridges are separated from the Flemish Banks by a narrow zone of deeper water. To the east they truncate the Zeeland ridges. To the north and west they are bordered by a flat or almost flat sea bottom. The ridges of the Hinder Group are elongated, isolated ridges between 17 km and 34 km long, which rise up to 30 m above the surrounding sea floor. Furthermore, the Hinder Banks are oriented anti-clockwise with respect to the tide (Houbolt, 1968). Dyer and Huntley (1999) classified these ridges as open shelf ridges.

Flemish Banks

Offshore of Flanders a complex series of ridges occurs, commonly known as the Flemish Banks. They consist of elongated ridges of 30-50 km length and up to 30 m height. The Flemish Banks show a strong degree of parallelism but they are connected in a complicated pattern, especially in the north. Van Veen (1936, in: Houbolt, 1968) attributes this complex morphology to a system of ebb and flood channels. Furthermore, the Flemish Banks are also oriented slightly anti-clockwise with respect to the tide. Dyer and Huntley (1999) classified these ridges also as open shelf ridges. In Table 3.1, a few parameters of the three sand ridge systems, Zeeland ridges, Hinder Banks and Flemish Banks are summarised.

Parameter Zeeland ridges Hinder Banks Flemish Banks

Height ridges 5-15 m 30 m 30 m

Length ridges 9-39 km 17-34 km 30-50 km

Orientation ridges with respect to tide

clockwise anti-clockwise anti-clockwise

From Table 3.1, we can see that the length of the Hinder Banks and Flemish Banks is of the same order of magnitude as that of the Zeeland ridges. Their height is however two times (or more) the height of the Zeeland ridges. Furthermore, the two systems on the BCS are oriented anti-clockwise with respect to the tide, whereas the Zeeland ridges are oriented clockwise with respect to the tide. Finally, Houbolt (1968) concluded that the Hinder Banks and the Flemish Banks were formed by sand accumulation, whereas he concluded that the Zeeland ridges were formed by erosion of older deposits. However, the origin of the Zeeland ridges is not clear (Section 3.3) (Houbolt, 1968).

Table 3.1: Characteristics Zeeland ridges, Hinder Banks and Flemish Banks

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3.7.2 Characteristics of the Zeeland ridges

In Table 3.2, an overview on the characteristics of the Zeeland ridges and the environment of the ridges is given.

Parameter Description Magnitude

Height ridges trough – crest 5 – 15 m

Length ridges 9 – 39 km

Width ridges half of distance trough to trough

2 – 3 km Transverse spacing between crests 3 – 7 km Orientation ridges with

respect to tide

clockwise 0 – 20 degrees

Bed forms sand waves 2 – 8 m (height)

Bottom slope < 1:1000

Tidal range: typical

spring (at Vlissingen) neap (at Vlissingen)

from north to south -2.0 to 2.5 m + NAP -1.0 to 1.5 m + NAP 2.5 – 3.5 m 4.5 m 2.5 m Tidal current : average

maximum at spring tide

at neap tide

0.75 m/s 1.0 m/s 0.6 m/s Grain size sediments:

- top of ridges - troughs

moderate coarse sand very coarse sand gravel silt 210 – 300 mm 300 – 420 mm 2 – 63 mm < 63 mm

Migration ridges Not observed

Sediment transport: - average total load - on top of ridges BCS and NCS Middelkerke bank 10 – 100 m3_/m/year 200 m3_/m/year

3.7.3 Conclusions

At first sight, the Zeeland ridges seem to be tidal sand ridges. According to their length and their height this would be the right conclusion. Comparing the Zeeland ridges to the Hinder Banks and Flemish Banks shows that they have about the same length, only they are not as high as the latter two groups of ridges. Furthermore, the orientation of the Zeeland ridges is not the same as that of the Hinder Banks and Flemish Banks, these are oriented anti-clockwise with respect to the tide like most tidal sand ridges. The Zeeland ridges are however oriented like shoreface-connected ridges, clockwise with respect to the tide. Except for the fact that the ridges are not connected to the shore. An explanation for the mirrored orientation can be found in the origin of the banks. According to the most recent research (Laban & Schüttenhelm, 1981) they were formed partly by erosion of older (dune) forms and partly by sand accumulation. So, this could be the explanation for the orientation of the ridges. However, no clear conclusion can be drawn on the classification of the Zeeland ridges.

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4 Information need

In this chapter, the information need of the different stakeholders, regarding sand extraction on the Zeeland ridges, will be specified (Section 4.1), which results in research questions and Coastal State Indicators (Section 4.2).

4.1 Information need stakeholders

4.1.1 Information need RWS (DNZ)

The information cycle, as described in Section 1.5, starts with a management question. From a Coastal Zone Management (CZM) perspective, like the perspective of DNZ, the context of sand extraction is determined by: the physical context (i.e. the physical- and ecological environment); the socio-economic context (i.e. socio-economic functional uses of the coastal zone) and the administrative context (i.e. the institutional arrangements, regulations, legislations and directives). Rational CZM will be based on an integrated analysis of this context (Website Sandpit, 2003). Thus, related to sand extraction the overall management question of DNZ is:

A rational coastal manager will aim for a rational decision-making process that is transparent and reproducible. The (vague) strategic CZM objectives must be translated into (specific) operational objectives. An important aid in this process is the definition of a set of Coastal State Indicators (CSI’s) (Soulsby, 2003). A CSI is defined as:

Each indicator is related to a specific coastal user function. Examples of coastal user functions are: coastal safety and maintenance, offshore infrastructure, navigation, recreation, ecosystem and fishery. For convenience, we have decided to restrict ourselves in this research to the first three coastal user functions and to the physical effects of sand extraction on them. Using this indicator-concept, the overall management question can be (re)phrased as:

What are the long-term effects of large-scale sand extraction on the Zeeland ridges?

What are the probable long-term physical effects, caused by large-scale sand extraction on the Zeeland ridges, which affect the coastal user functions: coastal safety and maintenance, offshore infrastructure and navigation?

A quantitative concept of the actual state of the system as a basis for objective and transparent decision-making; or a well defined physical and/or ecological variable quantifying a socio-economic functional use of the coastal zone.

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4.1.2 Information need RIKZ

RIKZ is engaged with all the salty and brackish waters in the Netherlands, and the coast. RIKZ is one of the six specialist departments of RWS. It gathers and provides knowledge and advice to support solving all kinds of sea related problems. In this research, RIKZ has two interests. Firstly, they want to gain (general) knowledge on how to deal with human interventions in the North Sea. Secondly, they need (specific) knowledge on the case of sand extraction on the Zeeland ridges and which instruments can be used to predict the physical effects, to advice RWS (DNZ).

4.1.3 Information need other stakeholders

Sandpit cycle

The Sandpit cycle (Figure 4.1, left) was developed for the European project Sandpit (see Website Sandpit) and presented at the RIKZ workshop (Hommes, 2004). The cycle starts with a request for sand extraction (mining). The first step in the cycle is to identify the problem, by determining stakeholders and investigating their concerns. Stakeholders define the spatial scale, which is the distance between the sand extraction and their concern. And the spatial scale determines the timescale of their concern. This is presented in the Scale graph (Figure 4.1, right). For example: a citizen is concerned that the beach will erode, the spatial scale of this topic is the coastline. Changes in the position of the coastline occur on the scale of years or decades, this is the timescale. The second step in the Sandpit cycle is to develop an assessment strategy with CSI’s and assessment criteria, which express the concerns of the stakeholders. The next step is to make predictions on the effects of sand extraction, using measurements and models. The last step of the cycle is to assess the predictions on the CSI’s and formulate a judgement, the decision on the request for sand extraction.

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Environmental information model

From the Sandpit cycle it follows that it is important to determine the information need of all stakeholders, to prevent conflicts., The environmental information model, which shows all the stakeholders involved in the problem, is given in Figure 4.2. We already determined the information need of the decision-maker (RWS, DNZ) and the water managers (RIKZ) in the previous sections. For the other stakeholders this is done briefly in this section. Furthermore, some points of concern are given; these are derived from workshops held by RWS in the framework of large-scale deep sand extraction (Van Woerden, 2002).

Environmental organizations

An example of an environmental organisation is ‘Stichting De Noordzee’ (SDN). SDN is non-governmental organisation that stands up for the North Sea and the ecosystem in particular. They aim at better regulations for the North Sea, expose misuse on the North Sea and communicate with governmental and non-governmental organisations about a clean and sustainable use of the North Sea. The public opinion is playing a key-role in these issues. The main points of concern for environmental organisations regarding the long-term effects of large-scale sand extraction are the following: insufficient recovery of sea bottom fauna (e.g. by oxygen deficit), extreme changes in algal communities (poisonous algae, etc.) and harmful effects on Bird- and Habitat areas (Van Woerden, 2002; Website SDN, 2003). In this research, we do not take the user function ecosystem into account. Therefore, these points of concern are not included.

Navigation

The Netherlands has one of the largest harbours of the world, Mainport Rotterdam. This generates a lot of traffic on the North Sea, which is regulated by deep-water routes and shipping channels. These routes and channels prevent conflicts between navigation and other user functions (Website Noordzee Atlas, 2003). The aim of the Dutch policy on sea navigation is to enlarge the sustainable additional value by the establishment of maritime activities in the Netherlands. Furthermore, the environment and safety are important issues of the Dutch policy on sea navigation. Finally, the use of the space on sea is an important issue. The points of concern for navigation regarding the effects of large-scale sand extraction focus on the issues: accessibility of harbours (sand accumulation in harbours by silt plumes) and safety (changes in water depths and currents) (Van Woerden, 2002; Website Noordzeeloket, 2003). The first issue focuses on the short-term and is therefore not included in this research. The point of concern safety is taken into account in the formulation of the CSI’s (Section 4.2).

Large scale sand extraction on sand ridges offshore of the Netherlands

Large-scale sand extraction

on sand ridges offshore of

the Netherlands

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Large-scale sand extraction

on sand ridges offshore of

the Netherlands

Preface

Table of Contents

List of figures

List of tables

List of abbreviations

Summary

Samenvatting

1 Introduction, Research objective and strategy

1.1 Sand extraction offshore of the Netherlands

1.2 Framework of the research

1.3 Previous research on offshore sand

extraction

1.4 Research objective

1.5 Research strategy

1.6 Reading guide

2 Rugby ball method

2.1 Rugby ball method in general

2.2 Rugby ball for this research

2.3 Exploration phase

2.3.1 Basic assumptions RWS (DNZ)

2.3.2 Information users sand extraction

2.3.3 Process information chain

2.4 Adjustment phase

3 Research area: Zeeland ridges

3.1 Classification ridges

3.1.1 Tidal sand ridges

3.1.2 Shoreface-connected ridges

3.2 Geomorphology of the Zeeland ridges

3.3 Origin of the Zeeland ridges

3.4 Sediments on the Zeeland ridges

3.5 Physical processes on the Zeeland ridges

3.5.1 Tides

3.5.2 Tidal currents

3.5.3 Tidal currents over ridges

3.5.4 Waves

3.5.5 Wind

3.5.6 Sediment transport

3.6 User functions in the area of the Zeeland ridges

3.7 Classification of the Zeeland ridges

3.7.1 Comparison with Belgian ridges

B

C

3.7.2 Characteristics of the Zeeland ridges

3.7.3 Conclusions

4 Information need

4.1 Information need stakeholders

4.1.1 Information need RWS (DNZ)

4.1.2 Information need RIKZ

4.1.3 Information need other stakeholders